These displays have a light box with light sources in a plurality of colours, which are lit successively and not simultaneously. The text that follows will refer only to the most conventional colours, red (R), green (G), blue (B), although other colours can be used in different distributions. Some displays use just two colours, for example green and red are involved equally. The pixels of the display do not have coloured filters. The image to be displayed is broken down electronically into three chromatic components corresponding to the three colours. The pixels are controlled as a function of the red component of the image to be displayed, the red light is turned on and then off, then the pixels are controlled as a function of the green component, the green light is turned on and then off, and finally the pixels are controlled as a function of the blue light and the blue light is turned on and then off. The rapid rate of these control and turn-on operations makes the human eye see a coloured image mixing with three components as if these three components were displayed simultaneously.
An image frame therefore comprises three subframes. In order to avoid a visual handicap due to a phenomenon referred to as “colour breakup”, the subframes can each be divided into two or three, that is to say that there would be two or three red subframes displaying the same image, two or three green ones and two or three blue ones, in one and the same frame. Each subframe comprises a phase of, row-by-row, writing of all the luminance levels of the points of the image for the colour under consideration; this is followed by a stabilization phase taking account of the reaction time of the liquid crystal; and this is followed by a turn-on phase for the source of the colour under consideration in the light box. For a frame of 16.67 milliseconds (operation at 60 Hz), with six subframes of 2.78 milliseconds, these three phases can typically last 0.7 millisecond, 1.07 milliseconds and 1 millisecond. The writing phase is broken down into writing of N successive rows; by way of example, N=256 and the duration reserved for writing a row is then shorter than 3 microseconds.
A colour sequential display therefore requires liquid crystals and control circuits with very fast switching, in the absence of which the displayed image is at risk of being flawed. The aim of the invention is to limit the switching speed constraints for colour sequential displays primarily, but the invention also applies to coloured pixel mosaic displays quite particularly when they need to display information (text, symbols, etc.) that is not video images.
In some colour sequential mode display applications, particular information is displayed in a first specific colour, whereas the background of the image is displayed in the other colour(s). This is the case, for example (but not exclusively), in displays in the field of avionics or other fields in which the display of important information is required in a manner superimposed on a background image.
To simplify the explanations, the example of a two-tone display will be taken in which critical information is displayed in a first red colour in a manner superimposed on a background image displayed in green, for example the image of a countryside. However, the invention also applies if the background image uses two or more colours, other than the first colour reserved for the information, and applies even if the background is black, as will be seen. By way of example, the information can be coloured symbols, or coloured text.